The evolution of cellular mobile communication networks beyond the third generation and the introduction of new broadband wireless access technologies such as WiMAX open the way to heterogeneous networks with a diversification of radio access technologies. In many cases providers offering the same RAT compete with each other on the market. Furthermore, a provider may offer services of different RATs to a user. As an example, a provider may provide UMTS (universal mobile telecommunications system) services as well as WLAN (wideband local area network) services. In such a situation the user is not interested in the particular technology, but is interested in getting the best quality of service (QoS) for his application. For that purpose it may be necessary to carry out a handover to another radio link with or without a change of the RAT.
In the prior art the desired application determines the RAT. If, for example, a high data rate is desired, e.g. for downloading music or a video file, WLAN is preferably used. If however, the user is interested in video telephony, UMTS is a good choice.
Providing the best radio link for an application requires in many cases vertical handovers, i.e. handovers from a communication system operating with a first RAT to a communication system operating with a second RAT. However, each RAT has its own QoS specifications as it has different modulation scheme and access schemes. As an example, a signal-to-noise-interference (SNI) ratio measured in a system using a first RAT, e.g. in a CDMA system, can hardly be compared with a SNI in a system using a second RAT, e.g. when a TDM link is established. As a consequence, the radio link quality in the new system cannot be anticipated and providing the best QoS for the user becomes difficult.
In the document A. Festtag, “Optimization of handover performance by link layer triggers in IP-based networks: parameters, protocol extensions and APIs for implementation”, TKN technical report TKN-02-014, TU Berlin, version 1.1, August 2002, two phases of a handover process are identified: a handover detection and triggering phase and a handover execution phase. In order to speed up the first phase the authors suggest the definition of a link layer trigger for a handover. A parameter for this link layer trigger is an abstract measure of a signal quality. This abstract measure is obtained by a mapping of RAT-specific measurement values into this abstract measure.